The two main systems of synthetic resins are thermosetting resins and thermoplastic resins. Thermosetting resins are normally in liquid state or are soluble or fusible at earlier stage of their manufacturing or processing, and after being subjected to a chemical change caused by heating, catalyzing or others (for example, ultraviolet lights, rays or the like), can be cross-linked to form insoluble and infusible resins having a tri-dimensional structure. Thermosetting resins include epoxy resins, unsaturated polyester resins, phenolic resins, amino resins, alkyd resins, silicone resins and the like. Among which, epoxy resins and unsaturated polyester resins are two representatives of three main thermosetting resins. Compared with thermoplastic resins, thermosetting resins have good heat resistance, high hardness and excellent electric performances and thus are widely employed for industrial and domestic purposes. However, thermosetting resins are inherently hard, but brittle, poorly crack-resistant and less tough after curing, which limit their applications in many areas. Therefore, many researches are focused on how to enhance toughness of thermosetting resins, so that they have more excellently balanced physical and mechanical properties.
There are mainly four methods for enhancing the toughness of thermosetting resins in the prior art: (1) a chemical modification method for enhancing the flexibility of the main chain; (2) a method for increasing the molecular weight of the polymerization monomer; (3) a method for decreasing the cross-linking density of thermosetting resins; (4) a method for adding toughening agents. Among which, the method for adding toughening agents is currently the most effective method for toughening thermosetting resins. Such a method was invented by Mc Garry and Willner in 1960s and they found that it was possible to substantially enhance the toughness of epoxy resins by mixing liquid carboxyl-terminated butadiene-acrylonitrile (CTBN) with epoxy resin prepolymers and then curing the resulting mixture under particular conditions. Recent tens of years, researches were focused on the effects of the molecular weight of CTBN, reactivity of the terminal groups, the content of nitrile, the interfacial adhesion between CTBN rubber and epoxy resin matrix, types and usage of curing agents, curing processes or the like on CTBN-toughened epoxy resins. In addition to toughening epoxy resins with CTBN, researches were made on toughening epoxy resins with other carboxyl-terminated rubbers (carboxyl-terminated polybutadiene (CTB), carboxyl-terminated styrene-butadiene rubber (CTBS), carboxyl-terminated polyether rubber (CTPE) or the like) and various hydroxy-terminated rubbers (liquid hydroxy-terminated nitrile rubber (HTBN), hydroxy-terminated polybutadiene (HTPB) or the like). See, for example, POLYMER TOUGHENING, Edit. CHARLES B. ARENDS, published by MARCEL DEKKER, Inc., p.131; Blending Modification of Polymers, Edits. WU Peixi and ZHANG Liucheng, published by China Light Industry Press, 1996, p.311; LI Ningsheng and SUN Zaijian, Science and Engineering of Polymer Materials, No. 5, p. 8-13(1987); YAN Hengmei, Applications of Engineering Plastics, No. 2, p.45-52(1989); Epoxy Resins, Edits. CHEN Ping and LIU Shengping, published by Chemical Industry Press, p.126-138(1999). Generally, when toughening thermosetting resins, the rubbers must meet the following conditions: (1) the rubber phases should well dissolved in the uncured resin system, and undergo phase separation during the cross-linking of the resins to form rubber microdomains dispersed in the resin matrix in a particular particle size. The controlling of phase separation will impose direct effects on the particle size of rubber phases. Therefore, when toughening thermosetting resins as in the prior art, there are a great restriction on the selection of rubber phases, a high requirement for the process control, and furthermore, a relatively complicated operation. (2) For producing a toughening effect to a certain extent, the rubbers must contain, in the molecular structures, reactive groups having reactivity with the resin matrix, in order to provide chemical bonds or a good compatibility between rubber phases and the matrix. (3) For producing a toughening effect to a certain extent, the rubbers are normally used in a relatively large amount, which will decreases heat resistance and strength of thermosetting resins. To sum up, there are four main problems when toughening thermosetting resins with liquid rubbers: (1) thermosetting resins normally have high heat distortion temperature and glass transition temperature and when toughening with rubbers having a low glass transition temperature, the toughened thermosetting resins normally have largely decreased heat distortion temperature and glass transition temperature, which results in a decrease in heat resistance of the articles and a decrease in strength of thermosetting resins. (2) The toughening effect on such thermosetting resins as unsaturated polyester resins or the like is not marked; (3) The rubbers have an unstable particle size, and the micromorphology of the toughened thermosetting resins is difficult to control, which result that the performances of the articles can not be easily reproduced. (4) When crazes rapidly grow (as in a standard Izod impact test), there will be no substantial toughening effect.
To overcome the above-mentioned disadvantages, many attempts are made and among which, the most effective one is a method for toughening epoxy resins with rubber particles having a core-shell structure. Such a method can enhance the toughness of epoxy resins, while the glass transition temperature of epoxy resins remains unchanged. However, such a toughening method fails to effectively solve the problem that when crazes rapidly grow (as in a standard Izod impact test), there will be no substantial toughening effect. Moreover, the fact that the glass transition temperature of resins remains unchanged does not demonstrate that the heat distortion temperature, i.e., the heat resistant temperature, does not decrease, thus such a method cannot impart epoxy resins with a relatively high heat distortion temperature.
Further, attempts were recently made on the modification of thermosetting resins with thermoplastic resins having a high heat resistance (for example, polyethersulfone (PES), polyetherimine resins (PEI), polyarylethersulfone (PSF) having terminal functional groups, or the like), in order to overcome the decrease in heat resistance of thermosetting resins caused by modification with rubbers. Although such systems contribute to the enhancement of heat resistance, the toughening effect is not desirable. See, for example, Blending Modification of Polymers, Edits. WU Peixi and ZHANG Liucheng, published by China Light Industry Press, 1996, p.311; H. KISHI, Y-B. SHI, J. HUANG, A. F. YEE, JOURNAL OF MATERRIALS SCIENCE 32(1997) 761-771.
Therefore, a bottleneck in the art is how to enhance the toughness of thermosetting resins while remaining their heat resistance.